This invention relates to an adaptive pulsing system for controlling a motor which moves a device to a desired position. The invention is useful wherever precise position control is desired, and is particularly useful if it is required that the final position always be approached from the same direction. This requirement is found in systems for which the position information is derived from measurement of lead screw rotation, since slack in the driving mechanism can lead to an error in measurement.
In Sweeney et al. U.S. Pat. No. 4,312,033, issued Jan. 19, 1982, titled "Digital Motor Control for Positioning System," a control apparatus and method are disclosed which provide a significant advance in solving the problem of unidirectional approach to a destination position. The system of that application uses a method of repeatedly traversing a fraction of the distance remaining to the destination, until the destination is reached. This system has proven itself highly successful, but it has been found that greater accuracy is attainable through a different method, described in Sweeney U.S. Pat. No. 4,353,019, issued Oct. 5, 1982, and titled "Adaptive Pulsing Motor Control for Positioning System."
U.S. Pat. No. 4,353,019 discloses a method of making a terminal approach to a destination by sending to the motor a series of short pulses of varying width. After sending an initial pulse to the motor having a predetermined width (duration), a short delay is observed; and then the position of the driven element is compared to its previous, or "target", position. If the driven element has not been moved past the target position, the motor pulse width is increased by a predetermined increment; and the longer pulse is used to energize the motor. This procedure is repeated, gradually increasing the pulse width, until the driven element moves past the target position. This method has been applied in a digital positioning system for industrial guillotine-type cutters which is installed as a retrofit for older machines. Its adaptive nature allows attainment of exceptional positioning accuracy even on machines with gross mechanical defects.
Although the system just described has proven very effective and accurate, there are advantages to be gained by using the system disclosed in the present application.
The use of a variable duration pulse width, i.e., a variable length delay after turning the motor on, has deficiencies when applied to a multiple-axis control system. Such a system ordinarily is implemented by "time-sharing" the central processor among the axes, which are independently approaching destination positions. If one axis were to appropriate the processor for an indefinite period, while another axis was making a high-speed approach, there would be a substantial likelihood that the latter axis would overshoot its mark. Another conflict could occur if two or more axes were in the final adaptive approach procedure, and if each were to add its pulse-on delay time to the other's pulse-off delay time. The pulse width for each axis could increase without limit, without altering the "duty cycle", i.e., the ratio of "on" time to the total of "on" time plus "off" time. While the system of U.S. Pat. No. 4,359,019 could be redesigned to permit time-sharing, the changes would be complex and costly.
Another deficiency in the system of that application is the constraint on the method of varying pulse width caused by the dual-voltage design of the motor drive circuit, which provides a high DC voltage to the motor for fast travel, and a low DC voltage for slow travel.
Additionally, the control apparatus and method of the present invention has proved to be even more accurate than the prior systems, and to be more rapid in approaching the destination position.
Another benefit of the present system is greater adaptability to the frictional differences which may be encountered in different positions of the driven element. In other words, there is greater certainty that motion will occur at some point during each pulsing cycle.